CHANGES IN MICROFOSSILS AND GEOCHEMISTRY THROUGH MID-CRETACEOUS OCEANIC ANOXIC EVENT 2 AT WESTERN NORTH ATLANTIC IODP SITE U1407

Benthic foraminifera are useful in reconstructing bottom-water oxygen conditions, yet they are usually scarce or absent during Oceanic Anoxic Event 2 (OAE2). However, at Site U1407, they are well preserved and abundant (2,500 individuals/g), providing a unique opportunity to reconstruct paleoceanographic changes throughout OAE2. This study couples counts of benthic foraminifera (BFN) and radiolarians together with geochemical proxies including stable isotopes of organic carbon (δ¹³C_org), total organic carbon (TOC), total inorganic carbon (TIC), phosphorus (P) and redox sensitive trace element (RSTE) content from sediments cored at IODP Site U1407. The results address the response of the western North Atlantic to this global event, and in particular assess the mechanisms behind organic matter (OM) accumulation.

At Site U1407, a negative excursion in δ¹³C_org precedes the global positive shift that marks the onset of OAE2. This positive δ¹³C_org increase is separated into two distinct peaks. The lower peak is characterized by variable TOC (0.3-3%) and relatively high BFN (300-900/g), but scarce radiolarians (5-130/g). The upper peak, the interval of maximum OM accumulation (16%), corresponds to a sharp increase in radiolarians (15,000/g), peaks in P and RSTE content, and sparse BFN (14/g). The higher OM accumulation is associated with increased productivity stimulated by upwelling and resulting in lower oxygen content, as shown by BFN and RSTEs. In contrast, OAE2 sediments above the upper δ¹³C_org peak are marked by a sharp negative inflection of 4.1‰, an increase in BFN (2,500/g), and coincident decreases in radiolarians, TOC, P and RSTEs, suggesting intermittent improvement in bottom ventilation. Similar episodes of high BFN and coeval negative excursions in δ¹³C_org during OAE2 are reported for the European “Plenus Cold Event” and Western Interior Seaway “Benthonic Zone”, attesting to the global extent of this minor perturbation.

In summary, these results indicate that at Site U1407, OM accumulation during OAE2 was mainly controlled by increased productivity and aided by the resulting diminished oxygenation.